1. Field of the Invention
The present invention relates to a driving technique for a stepping motor.
2. Description of the Related Art
In many cases, various kinds of electronic devices such as digital still cameras, digital video cameras, disk apparatuses, printers, copiers, and the like employ stepping motors for various kinds of operations such as adjustment of the position of movable components such as lenses, pickup heads, print heads, etc., driving operations for driving a sheet feeder roller, and the like. Such a stepping motor is configured as a synchronous motor that rotates in synchronization with an input pulse applied by an external circuit. The stepping motor has high controllability for starting, stopping, and positioning. Furthermore, the stepping motor can be driven using an open loop control method. In addition, the stepping motor has an advantage of high compatibility with digital signal processing.
For example, related techniques have been disclosed in Japanese Patent Application Laid Open No. H09-103096, Japanese Patent Application Laid Open No. 2004-120957, Japanese Patent Application Laid Open No. 2000-184789, and Japanese Patent Application Laid Open No. 2004-180354.
In the normal state, the rotor of the stepping motor rotates synchronously in increments of a step angle in proportion to the number of input pulses. However, when a sudden change in the load or a sudden change in speed occurs, the stepping motor goes out of synchronization. Such a state will be referred to as “step-out”. Once the stepping motor steps out, a special operation is required in order to restore the stepping motor to a normal driving state. Thus, there is a demand for a technique for preventing step-out.
In order to solve such a problem, in many cases, a motor driving circuit is designed such that a margin is applied to the assumed maximum load, thereby providing an output torque giving consideration to a margin for preventing step-out. However, such an arrangement has a disadvantage of large power loss. As another method, if a sensor is used to detect the position of the rotor, it leads to an increased cost due to an increased number of components.